Magnetic random access memory array with thin conduction electrical read and write lines

ABSTRACT

An MTJ MRAM cell is formed between ultra-thin orthogonal word and bit lines of high conductivity material whose thickness is less than 100 nm. Lines of this thickness produce switching magnetic fields at the cell free layer that are enhanced by a factor of approximately two for a given current. Because the lines require thinner depositions, there is no necessity of removing material by CMP during patterning and polishing. Therefore, there is a uniform spacing between the lines and the cell free layer.

This is a divisional application of U.S. patent application Ser. No.10/892,668, filed on Jul. 16, 2004, now issued as U.S. Pat. No.7,067,330.

RELATED PATENT APPLICATIONS

This Application is related to Ser. No. 10/910,725, filed on Aug. 3,2004 and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the design and fabrication of magnetic tunneljunctions (MTJ) as memory storage devices, particularly to a designwherein word and bit lines are ultra-thin so as to produce highermagnetic flux at the MTJ free layer for a given write current.

2. Description of the Related Art

The magnetic tunnel junction (MTJ) basically comprises two electrodes,which are layers of magnetized ferromagnetic material, separated by atunnel barrier layer, which is a thin layer of insulating material. Thetunnel barrier layer must be sufficiently thin so that there is aprobability for charge carriers (typically electrons) to cross the layerby means of quantum mechanical tunneling. The tunneling probability isspin dependent, however, because it depends on the availability oftunneling states that accept electrons with different spin orientations.Therefore, the overall tunneling current will depend on the number ofspin-up vs. spin-down electrons, which in turn depends on theorientation of the electron spin relative to the magnetization directionof the ferromagnetic layers. Thus, if the relative magnetizationdirections are varied for a given applied voltage, the tunneling currentwill also vary. As a result of this behavior of an MTJ, sensing thechange of tunneling current for a fixed voltage can enable adetermination of the relative magnetization directions of the twoferromagnetic layers that comprise it.

The use of an MTJ as an information storage device requires that themagnetization of at least one of its ferromagnetic layers can be variedrelative to the other and also that changes in these relative directionscan be sensed by means of variations in the tunneling current or,equivalently, the junction resistance. In its simplest form as a twostate memory storage device, the MTJ need only be capable of having itsmagnetizations put into parallel (low resistance) or antiparallel (highresistance) configurations, when writing data, and of having itstunneling current variations or resistance variations measured, whenreading data.

In practice, the free ferromagnetic layer of the MTJ can be modeled ashaving a magnetization which is free to rotate but which energeticallyprefers to align in either direction along its easy axis (the directionof magnetic crystalline anisotropy). The magnetization of the fixedlayer may be thought of as being permanently aligned in its easy axisdirection. When the free layer is anti-aligned with the fixed layer, thejunction will have its maximum resistance, when the free layer isaligned with the fixed layer, the minimum resistance is present. Intypical MRAM circuitry, the MTJ devices are located at the intersectionof orthogonal current carrying lines called word lines and bit lines.When both lines are carrying current, the device is written upon byhaving the magnetization direction of its free layer changed. When onlyone line is carrying current, the resistance of the device can besensed, so the device is effectively read. Note that an additionalcurrent carrying line may be present in some device configurations tosense the resistance of the device, but in simplest terms the devicebehaves as described above. Such an MTJ device is provided by Gallagheret al. (U. S. Pat. No. 5,650,958), who teach the formation of an MTJdevice with a pinned ferromagnetic layer whose magnetization is in theplane of the layer but not free to rotate, together with a free magneticlayer whose magnetization is free to rotate relative to that of thepinned layer, wherein the two layers are separated by an insulatingtunnel barrier layer.

In order for the MTJ MRAM device to be competitive, in terms of powerconsumption and device density, with other forms of DRAM, it isnecessary that the MTJ be made very small, typically of sub-microndimension. Parkin et al. (U. S. Pat. No. 6,166,948) teaches theformation of an MTJ MRAM cell in which the free layer is formed of twoantiparallel magnetized layers separated by a spacer layer chosen toprevent exchange coupling but to allow direct dipole coupling betweenthe layers. The free layer thereby has closed flux loops and the twolayers switch their magnetizations simultaneously during switchingoperations. Parkin notes that sub-micron dimensions are needed to becompetitive with DRAM memories in the range of 10-100 Mbit capacities.Parkin also notes that such small sizes are associated with significantproblems, particularly super-paramagnetism, which is the spontaneousthermal fluctuation of magnetization in samples of ferromagneticmaterial too small to have sufficient magnetic anisotropy (a measure ofthe ability of a sample to maintain a given magnetization direction). Toovercome the undesirable spontaneous thermal fluctuations in MRAM cellswith very small cross-sectional areas, it is necessary to make themagnetic layers thick. Unfortunately, the size of the required switchingfield increases with layer thickness, so the price paid for a thermallystable cell is the necessity of expending a great deal of current tochange the magnetic orientation of the cell's free layer.

Some degree of anisotropy is necessary if an MTJ cell is to be capableof maintaining a magnetization direction and, thereby, to effectivelystore data even when write currents are zero. As cell sizes havecontinued to decrease, the technology has sought to provide a degree ofmagnetic anisotropy by forming cells in a wide variety of shapes (eg.rectangles, diamonds, ellipses, etc.), so that the lack of inherentcrystalline anisotropy is countered by a shape anisotropy. Yet this formof anisotropy brings with it its own problems. A particularlytroublesome shape-related problem in MTJ devices results fromnon-uniform and uncontrollable edge-fields produced by shape anisotropy(a property of non-circular samples). As the cell size decreases, theseedge fields become relatively more important than the magnetization ofthe body of the cell and have an adverse effect on the storage andreading of data. Although such shape anisotropies, when of sufficientmagnitude, reduce the disadvantageous effects of super-paramagnetism,they have the negative effect of requiring high currents to change themagnetization direction of the MTJ for the purpose of storing data.

One way of addressing the problem of the high currents needed to changethe magnetization direction of a free layer when its shape anisotropy ishigh, is to provide a mechanism for concentrating the fields produced bylower current values. This approach was taken by Durlam et al. (U.S.Pat. No. 6,211,090 B1) who teach the formation of a flux concentrator,which is a soft magnetic (NiFe) layer formed around a copper damascenecurrent carrying line. The layer is formed around three sides of thecopper line which forms the digit line at the underside of the MRAMcell.

Two additional approaches are taught by Nakao, (U.S. Pat. No. 6,509,621B2), who, in one embodiment, forms a pinned layer out of a material thatproduces a high percentage of spin polarized electrons in the currentand in another embodiment, applies an offsetting magnetic field producedby a magnetic shield to assist in causing field reversals in the cellelement.

Yet another approach is taught by Sekiguchi et al., (U.S. Pat. No.6,611,455 B2) who forms free layers with easy axes that areperpendicular to the layer plane, then forms word and bit lines in thesame plane.

The present invention also addresses the problem of reducing the highcurrent required to reorient the magnetization of the free layer inultra-small MRAM cells wherein the super-paramagnetic behavior requiresthick free layers. It does so by forming word and/or bit lines ofexceptional thinness, specifically under 100 nm, compared toconventional thicknesses of approximately 300 nm, thereby increasing theswitching field at the free layer by as much as a factor of 2. Anadditional benefit of such thin lines is the ease of their fabrication.The patterning process for their formation requires the removal of lessmaterial and can eliminate the need for chemical mechanical polishing(CMP), which can produce uncontrollable variations in line thicknesses.Finally, these ultra-thin lines are easily formed into a variety ofconfigurations with respect to their positions relative to the MTJ cell.In the present invention the cell is positioned between the word and bitlines. In the following description the general method of forming thelines will be described along with illustrations of their placementrelative to the cell.

SUMMARY OF THE INVENTION

A first object of this invention is to provide an MTJ MRAM cell thatmakes more efficient use of word and bit line switching currents, thelines thereby producing magnetic fields of sufficient intensity forswitching, while requiring lower currents to do so.

A second object of this invention is to provide a method for fabricatingsuch a cell and its word and bit lines that simplifies the fabricationprocess and, in particular, eliminates uncontrollable variationsassociated with the process of chemical-mechanical polishing (CMP).

A third object of this invention is to provide such cells and arrays ofsuch cells.

These objects will be achieved by an MRAM cell design and fabricationmethod in which word and/or bit lines are formed of highly conductivematerials to an exceptional thinness of less than 100 nm. The conductingmaterial can be high conductivity materials such as Al, Cu, Au, Ru, Ta,CuAu, CuAg, NiCr, Rh and multilayers of these materials such as multiplelaminations of (NiCr/Cu).

In conventional word and bit lines of the prior art, the aspect ratio(ratio of thickness, t, to width w) of word/bit lines is close to one.Applying simple physics (Ampere's Law) indicates that the magneticfield, H_(s), at the surface of a prior art line of comparablethickness, t, and width, w, carrying current I is given by: H_(s)=πI/w.

For the proposed ultra-thin word/bit line design, where w>>t, themagnetic field obeys the following relationship: H_(s)=2πI/w. Thus,there is an enhancement of the magnetic field at the wire surface by afactor of two. Since the MRAM cell is located a small distance from theline surface, the benefit will be somewhat reduced, but will still besubstantial, particularly since the fabrication method allows thedistance between line and cell to be kept uniformly small.

FIG. 1 a shows a vertical cross-sectional view of the MTJ MRAM cell ofthe present invention. The multilayered cell element, whose horizontalcross-section is substantially circular, is at a vertically separatedintersection of an ultra-thin write word line (10) extending in lengthalong the z-axis of the illustrated axes (its width being in thex-direction and its thickness in the y-direction), and an ultra-thin bitline (20) extending in length along the x-axis (its width being in thez-direction and its thickness being in the y-direction) and verticallybelow the word line. These lines are used to perform write operations onthe cell, i.e. to switch the magnetization of its free layer. The writeword line and bit lines are separated by an insulating layer (15) andare also surrounded peripherally by insulation. The write word and bitlines have thicknesses, t_(w) and t_(b) respectively that aresubstantially less than their widths, w_(w) and w_(b) (not shown) inaccord with the objects of the present invention, wherein t_(w) andt_(b) are both less than approximately 100 nm and their widths arebetween approximately 300 and 500 nm. It is once again noted that wordand bit lines of the prior art are formed with comparable widths andthicknesses, both being between approximately 300 and 500 nm. Adescription of the ultra-thin write word or bit line fabrication processwill be described more fully below.

An MTJ MRAM cell element (50), of thickness between approximately 200and 400 angstroms and lateral dimension between approximately 0.3 and0.7 microns, is shown positioned between the intersection of the word(20) and bit lines (10). The cell element is a horizontal multi-layerfabrication comprising a seed layer (51), formed on the bit line, anantiferromagnetic pinning layer (52), a synthetic ferrimagnetic pinnedlayer which includes a second (53) and first (55) ferromagnetic layerseparated by a coupling layer (54), a tunnel barrier layer (56), a freelayer (57), which can also be a laminated structure, and a capping layer(58) beneath the bit line. An additional conducting electrode, called aread word line (59), used in read operations, is formed on an uppersurface of the cell. The word line is separated by insulation (15) fromthe conducting electrode. It is also possible to eliminate the electrodeand have the word line electrically contact the upper cell surface. Adetailed description of materials and dimensions used in the cell and inthe fabrication process will be discussed below within the descriptionof the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic vertical cross-sectional view of an MTJ MRAMcell having its cell element formed between ultra-thin word and bitlines of the present invention.

FIG. 1 b schematically shows a vertical cross-sectional view of an MTJMRAM cell of the present invention in an alternative configuration tothat in FIG. 1 a.

FIG. 1 c is a schematic drawing of an array (two being illustrated) ofMTJ MRAM cells formed between ultra-thin word and bit lines.

FIG. 1 d schematically shows an array of two cells of the type in FIG. 1a.

FIGS. 2 a-2 e provide a more detailed schematic description of theformation of an ultra-thin word or bit line, indicating how the thinnessof the word and bit lines makes their formation simpler.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention is an MTJ MRAM cellformed at an intersection of ultra-thin word and bit lines, morespecifically between those lines, so that smaller currents can stillproduce adequate switching fields at the location of the cell freelayer.

Referring again to FIG. 1 a, there is shown the multi-layered MTJ cellelement (50) formed between orthogonally directed, vertically separatedultra-thin word (10) and bit (20) lines of the invention. The two linesextend in vertically separated horizontal planes, crossing over eachother but insulated from each other and forming, thereby, anintersection at which the cell is located. In all that follows, the term“intersection” is taken to mean a vertically separated crossing oflines. The word line is directed perpendicularly out of the figureplane, the bit line is within the figure plane. An additional conductingelectrode, (alternatively denoted a read-word line), used in readoperations (59), is formed on an upper surface of the cell. Duringoperation of the cell, the conducting electrode will normally beconnected to an accessing transistor which is used to determine thelogic state of the MRAM cell. The electrode is insulated from andseparated from the word line, but it is clear that the separation shouldbe kept as small as possible to maintain the field strength of the wordline at the free layer of the cell. The bit line (20) can be formed as asingle layer of high conductivity material such as Al, Cu, Au, Ru, Ta,CuAu, CuAg, NiCr, Rh and multilayers of these materials such as multiplelaminations of (NiCr/Cu), formed to a thickness of approximately 100 nmor less and a width between approximately 300 and 500 nm. As previouslynoted, an additional conducting electrode (59), called a read word line,extending along the x-direction is formed contacting the upper surfaceof the cell and is used in conjunction with the bit line (20) for readoperations. A connecting line (60) goes from the electrode (59) to aterminal of an accessing transistor (not shown), which is a part of thecircuitry used to determine the logic state (i.e. its resistance) of theMRAM cell. In the particular configuration shown in FIG. 1 a, a singleMRAM cell is connected to a single transistor. A partial array of twosuch cells, an example of which is shown in FIG. 1 d, each formedbetween vertically separated, intersecting word and bit lines and eachaccessed by its own transistor, would form part of a particular MRAMarray design.

Referring back to the cell element (50), a seed layer (51) is formed onthe bit line (20) and promotes the high quality formation ofsubsequently formed layers of the cell. The seed layer can be a layer ofNiCr or NiFe formed to a thickness between approximately 5 and 100angstroms. A single pinned layer or, as in this embodiment, a syntheticferrimagnetic pinned layer is formed on the seed layer. The syntheticlayer includes an antiferromagnetic pinning layer (52), a secondferromagnetic layer (53), a coupling layer (54) and a firstferromagnetic layer (55). The antiferromagnetic layer pins themagnetization of the second ferromagnetic layer unidirectionally and thefirst ferromagnetic layer is magnetized in an antiparallel direction tothat of the first layer. The first and second ferromagnetic layers arelayers of CoFe formed to thicknesses between approximately 5 and 100angstroms and matched so that the net magnetic moment of theconfiguration is substantially zero. The coupling layer is a layer ofRh, Ru, Cr or Cu of proper thickness to maintain strong antiparallelcoupling between the magnetic moments. The antiferromagnetic pinninglayer (52) can be a layer of PtMn, NiMn, OsMn, IrMn, PtPdMn, PtCrMn orFeMn of thickness between approximately 10 and 500 angstroms.

A tunneling barrier layer (56) is formed on the first ferromagneticlayer (55) of the pinned layer. This layer is a layer of insulatingmaterial such as oxidized Al or an oxidized Al—Hf bilayer and is formedto a thickness between approximately 7 to 15 angstroms. A ferromagneticfree layer (57) is formed on the barrier layer. The free layer can be asingle layer of ferromagnetic material, such as a layer of CoFe or NiFeformed to a thickness between approximately 5 and 100 angstroms or itcan be a multilayer, comprising first and second ferromagnetic layers,magnetized in antiparallel directions and separated by a spacer layer ofnon magnetic but conducting material such as Rh, Ru, Cr or Cu, which isof the proper thickness to maintain strong antiparallel coupling betweenthe two ferromagnetic layers. During formation of the cell it isadvantageous to set the magnetic anisotropy direction of theferromagnetic layers either perpendicular or parallel to the bit line. Acapping layer (58) is formed on the free layer and completes the cellelement (50). The capping layer can be a layer of Ru, or Ta or amultilayer of Ru/Ta formed to a thickness between approximately 5 and100 angstroms. The read word line (59) is formed on the capping layer(58) of the cell element (50). A layer of insulating material (15)surrounds the cell and separates the write word line (10) from the bitline (20) and the read word line (59) from the word line (10). The writeword line (10), like the bit line, is an ultra-thin layer of conductingmaterial less than 100 nm in thickness formed in accord with the methodof this invention. It is further noted that the separation between theread and write word lines must be kept as small as possible to maintainthe strength of the write word line magnetic field at the cell elementfree layer. A separation no greater than the thickness of the ultra-thinlines is preferable.

Referring to FIG. 1 b, there is shown an MTJ MRAM design employing theultra-thin word (10) and bit (20) lines of the present invention, butdiffering from the design of FIG. 1 a in that the read word line ((59)in FIG. 1 a) is absent and, instead, the write word line (10) is formedcontacting the upper surface of the cell element (50). The cell elementis identical to the cell element in FIG. 1 a, so it is not drawn indetail. In this configuration, the logic state of the cell is determinedusing only the write word line and bit line.

Referring to FIG. 1 c, there is shown, schematically, an array of MRAMcells, in which a plurality of cell elements (50) (two being shown),each element being identical to the cell element in FIG. 1 a, is formedbetween a common word line (10) and an individual bit line (20), in theconfiguration of FIG. 1 b. The word line is then connected to a singleaccessing transistor (not shown) by a connecting conductor (60). Eachcell, of which only two are shown for clarity and both are labeled (50),is formed contacting a separate bit line, labeled (20), which isdirected out of the plane of the figure. The upper surface of the cellelement, which in this configuration is the capping layer (58), contactsthe lower surface of the bit line (20) and the lower surface of the cellelement, which is the seed layer (51), contacts the upper surface of theword line (10). There is no separate electrode, such as that denoted by(59) in FIG. 1 a. The word line (10) and the bit lines (20) are formedin accord with the method of the invention to be described below withreference to FIGS. 2 a-2 d. In this configuration, all the cellscontacting the common word line are accessed by a single transistor. Itis noted that this array can be inverted, so that the word line is abovethe bit lines, the cell element has its layers inverted and theaccessing transistor is above the word line.

Referring to FIG. 1 d, there is shown an array of two MTJ MRAM cells,each of the configuration of FIG. 1 a, wherein each cell element (50) isplaced between intersecting word (10) and bit (20) lines formed usingthe method of the present invention and wherein the same bit line (20)is common to each cell but each word line is above an individual cellelement. An electrode (59) is formed contacting the upper surface ofeach cell on its capping layer (58) and insulated (15) from the wordline, and each electrode is connected to an accessing transistor (notshown) by a conducting line (60). In this array configuration there isone transistor for each cell. It is noted that the entire configurationmay be inverted, so that the bit line is vertically above the cell andthe cell layer structure is inverted relative to the illustration inFIG. 1 a.

Referring now to FIGS. 2 a-2 e, there are schematically shown several ofthe steps involved in fabricating the bit or word lines of the presentinvention. The extreme thinness of the lines not only accomplishes theobject of the invention, which is to provide an adequate switching fieldwith a smaller current, but they also can be fabricated in an easierfashion than conventional thicker lines since less ion-beam etch (IBE)trimming and CMP polishing is required.

Referring first to FIG. 2 a, there are shown the first of the processsteps required to form the ultra-thin word or bit lines of the presentinvention. First a thin conducting layer (100) is deposited over asubstrate (90) having a substantially planar upper surface, theconducting layer being deposited to the desired final thickness of theword or bit line by a process of sputtering, ion-beam deposition (IBD)or chemical vapor deposition (CVD). It is noted that the substrate maybe a dielectric layer that includes MTJ MRAM cell elements whose uppersurfaces are co-planar with the upper surface of the dielectric layer.Alternatively the substrate may be a dielectric layer that is formedover a conducting electrode, such as (59) in FIG. 1 a. The substrate isshown here devoid of any detail. A layer of photoresist (200) is thenformed on the conducting layer.

Referring to FIG. 2 b, there is shown the photoresist layer nowpatterned (210) by a photolithographic process such as is well known inthe art. The patterning produces a strip (or a plurality of strips ifmore than one line is to be formed) that has the width of the line to beformed and extends in the proper line direction.

Referring to FIG. 2 c, there is shown the patterned photoresist (210)having been used as a mask for an ion-beam etch (IBE) or reactive ionetch (RIE), to remove peripheral portions of the conducting layer andleave behind the desired word/bit line (150) beneath the photoresistpattern. The photoresist will then be removed (not shown) to leave onlythe word/bit lines (150) properly arranged over the substrate (90).

Referring to FIG. 2 d, there is shown the formation of FIG. 2 c whereinan insulating refill layer (250) has been deposited to fill the spacesbetween the word/bit line just formed and any others (not shown). Inthis form, MTJ cells may be formed over the word/bit lines or anorthogonal set of word lines may be formed over these lines if they arebit lines. If the lines just formed (150) are bit lines (runningorthogonally to the figure plane), then the orthogonal lines formed overthem will be word lines (running within the figure plane).

Referring finally to FIG. 2 e, there is shown, just as an example, asubstrate (90) that includes an MTJ MRAM cell (50), so that theultra-thin word line (150), formed in accord with FIGS. 2 a-2 d, ispositioned on the upper surface of the cell. It should be clear to oneskilled in the art how other ultra-thin intersecting word/bit lineconfigurations can be formed at the location of cell elements.

For uniformity and reproducibility it is required that the surfaces ofthe bit or word lines (150) not be smoothed or reduced in thickness by aprocess step of chemical mechanical polishing (CMP). Such polishing willintroduce undesirable thickness variations in the lines, whichvariations, in turn, will adversely affect the maintaining of asufficiently small and uniform distance between the lines and the freelayer within the MTJ cell. The thickness variations result because ofthe difficulty in controlling the CMP lapping process and obtaining anaccurate stopping point. Since CMP is therefore ruled out, the bit linescannot be made thick, because a thick deposition will inevitably have ahighly non-planar upper surface, which, without CMP, is then a cause ofsubsequent problems, including inaccurate photoresist patterning, poorline continuation and electro-migration. Thus, the thin deposition ofthe present invention eliminates the requirement of a disadvantageousCMP process and simultaneously provides the increased magnetic fieldsrequired.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than being limiting of the present invention. Revisionsand modifications may be made to methods, processes, materials,structures, and dimensions through which an MTJ MRAM cell, comprising acell element between an ultra-thin bit line and an ultra-thin word lineis formed and provided, while still forming and providing such an MRAMcell, in accord with the present invention as defined by the appendedclaims.

1. An MTJ MRAM cell formed at an intersection of ultra-thin word and bitlines comprising: an ultra-thin bit line, having an upper surface and alower surface and formed of high conductivity material, extending in afirst direction within a first horizontal plane and having a thickness,t_(b), that is less than 100 nm; an ultra-thin write word line, havingan upper surface and a lower surface and formed of high conductivitymaterial, said write word line extending in a direction orthogonal tosaid first direction in a second horizontal plane that is verticallyseparated from said first horizontal plane and wherein said ultra-thinwrite word line passes vertically over said ultra-thin bit line to forma vertically separated intersection with said ultra-thin bit line andsaid ultra-thin word line having a thickness, t_(w), that is less than100 nm; a horizontally multilayered magnetic tunnel junction (MTJ) cellelement, having an upper surface and a lower surface and including amagnetically free layer, formed at said intersection of said ultra-thinword line and said ultra-thin bit line.
 2. The MTJ MRAM cell of claim 1wherein a MTJ cell element is formed between said ultra-thin word andbit lines at their intersection and wherein the upper surface of saidMTJ cell element electrically contacts the lower surface of saidultra-thin word line and the lower surface of said MTJ cell elementelectrically contacts an upper surface of said ultra-thin bit line andwherein said ultra-thin word and bit lines produce enhanced magneticfields at said MTJ cell element free layer.
 3. The MTJ MRAM cell ofclaim 1 wherein a MTJ cell element includes a conducting electrodeformed on an upper surface.
 4. The MTJ MRAM cell of claim 3 wherein aMTJ cell element is formed between said ultra-thin word and bit lines attheir intersection and wherein a lower surface of said MTJ cell elementcontacts an upper surface of said ultra-thin bit line and wherein alower surface of said ultra-thin word line is vertically above saidconducting electrode and separated from said conducting electrode by alayer of insulation and wherein said ultra-thin word and bit linesproduce enhanced magnetic fields at said MTJ cell element free layer. 5.The MTJ MRAM cell of claim 1 wherein said high conductivity material isCu, Au, Al, Ag, CuAg, Ta, Cr, NiCr, NiFeCr, Ru, Rh or multiply laminatedlayers of said materials.
 6. The MTJ MRAM cell of claim 1 wherein saidMTJ cell element comprises: a seed layer formed on the upper surface ofsaid ultra-thin bit line; an antiferromagnetic pinning layer formed onsaid seed layer; a synthetic ferrimagnetic pinned layer formed on saidantiferromagnetic layer, said pinned layer comprising first and secondferromagnetic layers of substantially equal and opposite magneticmoments, separated by a first coupling layer; a tunneling barrier layerformed on said pinned layer; a ferromagnetic free layer formed on saidtunneling barrier layer; a capping layer formed on said ferromagneticfree layer; and wherein the magnetic anisotropy of the ferromagneticlayers is set parallel or perpendicular to said ultra-thin bit line. 7.The MTJ MRAM cell of claim 6 wherein the ferromagnetic free layer is asynthetic ferrimagnetic layer comprising third and fourth ferromagneticlayers of substantially equal and opposite magnetic moments, separatedby a second coupling layer of sufficient thickness to maintain a strongantiparallel coupling of said magnetic moments.
 8. The MTJ MRAM cell ofclaim 6 wherein the antiferromagnetic pinning layer is a layer of PtMn,NiMn, OsMn, IrMn, PtPdMn, PtCrMn or FeMn of thickness betweenapproximately 10 and 500 angstroms and the first and secondferromagnetic layers are layers of CoFe or NiFe formed to a thicknessbetween approximately 5 and 100 angstroms and the coupling layer is alayer of Rh, Ru, Cu or Cr of sufficient thickness to maintainantiparallel coupling of the ferromagnetic layers.
 9. An array of MTJMRAM cells formed between intersections of ultra-thin word and bit linesof high conductivity material, said array comprising: at least oneultra-thin word line, having an upper surface and a lower surface,formed in a first direction; a plurality of identical MTJ cell elementsformed, uniformly spaced, on an upper surface of said ultra-thin wordline, a lower surface of each said cell element electrically contactingthe upper surface of said ultra-thin word line; a plurality of parallelultra-thin bit lines, each of said ultra-thin bit lines having an upperand a lower surface and the lower surface electrically contacting anupper surface of one of said MTJ cell elements and each of saidultra-thin bit lines formed in a second direction that is orthogonal tosaid first direction; an electrically conducting connection formedbetween said ultra-thin word line and a single accessing transistor. 10.The array of MTJ MRAM cells of claim 9 wherein each said MTJ cellelement comprises: a seed layer formed on the lower surface of saidultra-thin bit line; an antiferromagnetic pinning layer formed on saidseed layer; a synthetic ferrimagnetic pinned layer formed on saidantiferromagnetic layer, said pinned layer comprising first and secondferromagnetic layers of substantially equal and opposite magneticmoments, separated by a first coupling layer; a tunneling barrier layerformed on said synthetic ferrimagnetic pinned layer; a ferromagneticfree layer formed on said tunneling barrier layer; a capping layerformed on said ferromagnetic free layer and contacting the upper surfaceof said ultra-thin word line; and wherein a magnetic anisotropy of thefirst and second ferromagnetic layers is set parallel or perpendicularto the ultra-thin bit line.
 11. An array of MTJ MRAM cells formedbetween intersections of ultra-thin word and bit lines formed of highconductivity material, said array comprising: at least one ultra-thinbit line having an upper and a lower surface formed in a first directionin a first plane; a plurality of parallel ultra-thin word lines, eachhaving an upper and a lower surface, formed in a second directionorthogonal to said first direction and in a second plane, parallel tosaid first plane and vertically above and separated from said firstplane; a plurality of identical MTJ cell elements formed on saidultra-thin bit line between each intersection of said ultra-thin bitline and said ultra-thin word lines, each MTJ cell element having anupper and a lower surface wherein the lower surface of each said MTJcell electrically contacts the upper surface of said ultra-thin bitline; a conducting electrode formed contacting the upper surface of eachMTJ cell, said conducting electrode being below the lower surface ofsaid ultra-thin word line and insulated from said ultra-thin word lineand said electrode being electrically connected to an accessingtransistor.
 12. The array of MTJ MRAM cells of claim 11 wherein eachsaid MTJ cell element comprises: a seed layer formed on the uppersurface of said ultra-thin bit line; an antiferromagnetic pinning layerformed on said seed layer; a synthetic ferrimagnetic pinned layer formedon said antiferromagnetic pinning layer, said synthetic ferrimagneticpinned layer comprising first and second ferromagnetic layers ofsubstantially equal and opposite magnetic moments, separated by a firstcoupling layer; a tunneling barrier layer formed on said syntheticferrimagnetic pinned layer; a ferromagnetic free layer formed on saidtunneling barrier layer; a capping layer formed on said ferromagneticfree layer; and wherein the magnetic anisotropy of the first and secondferromagnetic layers is set parallel to or perpendicular to theultra-thin bit line.